Electrospinning is a versatile method for generating very thin fibers made of polymers, ceramics, metals, carbon, and/or composite materials. A somewhat similar technique called electrospray can be used to produce a micro/nanometric jet that breaks up to give rise to an aerosol of charged droplets. Electrospray has a proven ability to generate monodisperse aerosols with sizes ranging from a few nanometers to hundreds of microns. Electrospinning, by contrast, typically generates a jet in a high-voltage field to produce elongated polymeric fibers. Compared with electrospray, which uses electro-hydrodynamic forces to generate a number of particles in an aerosol or a hydrosol phase, electrospinning is a more demanding technique that requires the use of a solution with appropriate viscosity, surface tension, and conductivity to produce continuous liquid jets.
In conventional electrospinning, a suitable polymer solution, or melt, is subjected to a high-voltage electrical field to create an electrically charged jet that typically dries or solidifies to create a solid fiber. For example, one electrode from a high-voltage source may be placed into a polymer solution and the other attached to a conductive collector, such as a panel of aluminum foil or a silicon wafer. A typical apparatus for electrospinning utilizes a spinneret with a metallic needle, a syringe pump providing the working fluid to the spinneret, a high-voltage power supply, and a grounded collector. A polymer, sol-gel, or composite solution or melt is loaded into the syringe pump, and this viscous liquid is driven to the needle tip, forming a droplet at the tip. When a voltage is applied to the metallic needle, the droplet is first stretched into a structure called a Taylor cone and, finally, into an electrified jet. The jet is then elongated and whipped continuously by electrostatic repulsion until it is deposited on the grounded collector. The elongation by bending instability results in the formation of uniform fibers that may typically have nanometer-scale diameters.
It is known that nanotubes and nanofibers with core-sheath, hollow, or porous structures have many promising applications in a wide variety of technologies including, for example, microfluidics, photonics, and energy storage. Prior art techniques have used self-assembly methods to build nanotubes from organic building blocks, although there are significant and well-known limitations to this approach. In addition, nanotubes have been fabricated from layered structures by carefully controlling the manufacturing conditions. By this method, nanotubes with diameters less than about 10 nm were created from materials, such as graphitic carbon, metal dichalcogenides, metals, and metal oxides. Fabrication of larger nanotubes and core-sheath nanofibers has been dominated by template-directed approaches. The yield and quality of the resultant material is generally dictated by the efficiency and efficacy of the coating and etching steps used. In particular, it is difficult to create core-sheath and hollow nanofibers of long length using prior art methods due to interconnections between fibers formed during the coating and etching steps.
Disclosed herein is an electrospinning process for the production of micro- and nano-scale core-sheath, hollow, or porous fibers (collectively, nanofibers).